A single-fed reconfigurable microstrip antenna that can provide various polarization diversities is presented in this paper. The antenna is excited by a microstrip feed line through aperture coupling. When two PIN diodes are used to respectively reconfigure the coupling slot and the open stub of the feed line, the polarization of the microstrip antenna can be switched between vertical and horizontal polarizations. For the reconfigurable antenna with the linear polarization diversity, it can be converted to a circularly-polarized antenna with switchable polarization sense by introducing a perturbation segment. Moreover, an antenna with the quadri-polarization diversity, including dual orthogonal linear polarizations and two circular polarizations, is also developed from the structure of the switchable circular polarization microstrip antenna, and only three diodes are required. Details of the antenna design are shown, and the measured results for the constructed prototype are also exhibited and discussed. View full abstract»

Analytical evaluation of asymptotic part of the Green's function, in product with piece-wise sinusoidal, entire domain, and travelling wave mode basis functions, is presented. The Sommerfeld type double integral in spectral domain is solved and reduced into a single integral in spatial domain with very short integration range. The derived results lead to develop a computational efficiency improved code, which has been found 27 times faster than the earlier validated common method of moments code. View full abstract»

An extrapolation technique is presented which reduces the computational demands of obtaining a wideband electromagnetic response from a resonant antenna using traditional computational electromagnetic methods. It has been shown that a wideband response can be extrapolated by fitting early-time and low-frequency data with a summation of orthogonal polynomials. However, representing responses characteristic of resonant structures in practice proves computationally inefficient and can lead to numerical instabilities. This paper outlines the incorporation of damped sinusoids to efficiently, accurately, and reliably extrapolate both time- and frequency-domain responses of resonant antennas due to a wideband source. A genetic algorithm is used to select the necessary extrapolation parameters. The wideband driving-point current response of several resonant antennas is accurately extrapolated. The transmission-line matrix method and the method of moments are used to compute early-time and low-frequency data, respectively. Fundamentally different discretizations of the structure of interest are used, illustrating in principle the independence of the technique and the choice of computational methods used to provide the directly-computed data. View full abstract»

A novel method is presented for electrically tuning the frequency of a planar inverted-F antenna (PIFA). A tuning circuit, comprising an RF switch and discrete passive components, has been completely integrated into the antenna element, which is thus free of dc wires. The proposed tuning method has been demonstrated with a dual-band PIFA capable of operating in four frequency bands. The antenna covers the GSM850, GSM900, GSM1800, PCS1900 and UMTS frequency ranges with over 40% total efficiency. The impact of the tuning circuit on the antenna's efficiency and radiation pattern have been experimentally studied through comparison with the performance of a reference antenna not incorporating the tuning circuit. The proposed frequency tuning concept can be extended to more complex PIFA structures as well as other types of antennas to give enhanced electrical performance. View full abstract»

The design of a multifrequency dipole antenna array based on a resonant meta-surface superstrate is proposed. The behavior of a single element that is closely placed to a meta-surface is experimentally investigated. The proposed meta-surface is based on resonating unit cells formed by capacitively loaded strips and split ring resonators. By tuning a dipole antenna to the pass band of the meta-surface, the physical area is effectively illuminated enhancing the radiation performance. The gain, radiation efficiency and effective area values of the whole configuration are compared to the ones obtained with a single dipole without superstrate. Radiation efficiency values for the proposed configuration of more than 80% and gain values of more than 4.5 plusmn 1 dB are obtained. Based on this configuration, simulated results of a multifrequency antenna array are presented. Distinctive features of this configuration are high isolation between elements (20 dB for a distance of lambda0/4), and low back radiation. View full abstract»

A formulation based on Lagrangian optimization and spheroidal vector wave functions is presented for the vector electromagnetic inverse source problem of deducing a time-harmonic current distribution that is confined within a spheroidal volume, that generates a prescribed radiation field, and that is subject to given constraints on the source functional energy, which characterizes antenna current level, and the source's reactive power, which models antenna resonance matching. The paper includes computer simulation results illustrating the derived inverse theory. View full abstract»

The intimate relationship is discussed between linear vector space concepts, the reciprocity theorem, and the energy conservation theorem in electromagnetics. It shows that these theorems can be derived using nonconventional methods. The reciprocity theorem can be derived by designing symmetric operators. The Lorentz reciprocity theorem can be derived by augmenting the operator with an extra term that generates equivalence surface currents. The energy conservation theorem for the real part of the complex power is derived by designing an operator that becomes Hermitian in the lossless case. The energy conservation theorem for the imaginary part of the complex power is derived by designing an operator which becomes skew Hermitian in the lossless case. View full abstract»

The results obtained in a previous paper are extended from bounded to unbounded spaces. To do so, a new term, simply structured, is introduced. The result is as follows. In a simply structured space, whether it is bounded or unbounded, a piecewise smooth divergence-free vector function of position is the curl of another vector function of position called a vector potential. With this extension, it becomes clear why one can claim the existence of magnetic vector potentials. Magnetic induction is piecewise smooth and divergence-free in a simply structured space called the universe. Seven examples are presented to facilitate the understanding of the present discussion. They are about the electric and magnetic fields in infinite straight coaxial cables and the electric field in modified Euclidean spaces. View full abstract»

A new technique for machine precision evaluation of singular and nearly singular potential integrals with 1/R singularities is presented. The numerical quadrature scheme is based on a new rational expression for the integrands, obtained by a cancellation procedure. In particular, by using library routines for Gauss quadrature of rational functions readily available in the literature, this new expression permits the exact numerical integration of singular static potentials associated with polynomial source distributions. The rules to achieve the desired numerical accuracy for singular and nearly singular static and dynamic potential integrals are presented and discussed, and several numerical examples are provided. View full abstract»

We describe a numerically efficient strategy for solving a linear system of equations arising in the Method of Moments for solving electromagnetic scattering problems. This novel approach, termed as the characteristic basis function method (CBFM), is based on utilizing characteristic basis functions (CBFs)-special functions defined on macro domains (blocks)-that include a relatively large number of conventional sub-domains discretized by using triangular or rectangular patches. Use of these basis functions leads to a significant reduction in the number of unknowns, and results in a substantial size reduction of the MoM matrix; this, in turn, enables us to handle the reduced matrix by using a direct solver, without the need to iterate. In addition, the paper shows that the CBFs can be generated by using a sparse representation of the impedance matrix-resulting in lower computational cost-and that, in contrast to the iterative techniques, multiple excitations can be handled with only a small overhead. Another important attribute of the CBFM is that it is readily parallelized. Numerical results that demonstrate the accuracy and time efficiency of the CBFM for several representative scattering problems are included in the paper. View full abstract»

The surface current for scattering from a PEC strip naturally decomposes into three slowly varying functions modulating rapidly oscillating phase factors. We exploit this structure to derive a numerical solution that is error-controllable and exhibits a bounded error over the full range of frequencies. Frequency independence is obtained by expanding the current in terms of slowly-varying amplitude functions, stretching coordinates in the boundary layer, and employing a frequency-independent quadrature rule. Though the total current solution is always well-defined, unique, minimally-varying amplitude functions may also be found by using the minimum norm concept. View full abstract»

We introduce a conformal perfectly matched layer (PML) for the finite-element time-domain (FETD) solution of transient Maxwell equations in open domains. The conformal PML is implemented in a mixed FETD setting based on a direct discretization of the first-order coupled Maxwell curl equations (as opposed to the second-order vector wave equation) that employs edge elements (Whitney 1-form) to expand the electric field and face elements (Whitney 2-form) to expand the magnetic field. We show that the conformal PML can be easily incorporated into the mixed FETD algorithm by utilizing PML constitutive tensors whose discretization is naturally decoupled from that of Maxwell curl equations (spatial derivatives). Compared to the conventional (rectangular) PML, a conformal PML allows for a considerable reduction on the amount of buffer space in the computational domain around the scatterer(s). View full abstract»

A general three-dimensional tensor finite-difference time-domain (TFDTD) formulation is derived to model electrically inhomogeneous lossy media of arbitrary shapes. The time domain representation of electric losses is achieved using Z-transforms. The regular cubical grid structure is maintained everywhere in the calculation domain by defining a 3-D face-fraction based 3 x 3 permittivity tensor on the interfaces that describes the relationship between the (known) average flux density vector and the (unknown) local electric field vector. For electrically lossy media, this tensor is complex in the frequency domain. However, it can be modified for use with the Z-transform. Only this modified real form is inverted, then transformed from the frequency into the Z-domain, and finally into the time domain. Furthermore, a local interface matrix is used to describe the relationship between the local electric field in the grid node and its counterpart on the other side of the interface. This matrix is complex in the frequency domain for lossy media. By applying the Z-transform, this matrix can also be transformed into the time domain using only real modified matrix elements. The accuracy of the method is confirmed by comparisons with analytical solutions. View full abstract»

The finite-difference time-domain (FDTD) method is successfully used to calculate the electric fields radiated from a prototype of a RJ45 connector for Ethernet transmission. Simulation results of the electric fields radiated from the prototype of this RJ45 connector are validated by measurement data for vertical and horizontal polarizations in 300 ~ 1000 MHz frequency range. It is also found that simulation and measurement results for both vertical and horizontal polarizations are over the regulation limit at 600 MHz. In order to meet regulation limits, further studies are also conducted by taking numerous possible affecting factors on radiated electric fields into consideration for FDTD simulations; these include the aperture area, the location of the excitation, the material used for the aperture, enclosures with and without grounding. From simulation results presented in this paper, it is found that the magnitude of the higher emission levels can be reduced by reducing the aperture area, putting the location of excitation deeper into the connector, using materials with smaller dielectric constants and conductivities for the aperture, and adding a grounding enclosure on the RJ45 connector. View full abstract»

We study the propagation of electromagnetic fields in forest media in a frequency band between 100 and 300 MHz. We develop an exact coherent model in order to compute the electromagnetic field of a received wave after multiple scattering from the elements of the medium, in and outside the forest. We propose simplifications for the model of the forest and approximations for the electromagnetic method to reduce the computing time. We use and simulate a DORT based method to detect and locate a hidden target in the medium. We develop this method to adapt it to a real configuration and we apply it to measurements in anechoic chamber. View full abstract»

We address the problem of tracking the spatial and temporal lower atmospheric variations in maritime environments. The evolution of the range and height-dependent index of refraction is tracked using the sea clutter return from sea-borne radars operating in the region. A split-step fast Fourier transform based parabolic equation approximation to the wave equation is used to compute the clutter return in complex environments with varying index of refraction. In addition, regional statistics are incorporated as prior densities, resulting in a highly nonlinear and non-Gaussian tracking problem. Tracking algorithms such as the extended Kalman, unscented Kalman and particle filters are used for tracking both evaporative and surface-based electromagnetic ducts frequently encountered in marine environments. The tracking performances and applicability of these techniques to different types of refractivity-from-clutter problems are studied using the posterior Cramer-Rao lower bound. Track divergence statistics are analyzed. The results show that while the tracking performance of the Kalman filters is comparable to the particle filters in evaporative duct tracking, it is limited by the high non-linearity of the parabolic equation for the surface-based duct case. Particle filters, on the other hand, prove to be very promising in tracking a wide range of environments including the abruptly changing ones. View full abstract»

Propagation of radio signals from a base above clutter, such as buildings and trees, to a mobile immersed in clutter is treated theoretically by accounting for random diffuse scattering at the mobile. Small-scale fading and distance-dependent loss are treated in a unified way, as opposed to the heuristic methodologies, which formulate them as separate factors. Closed form expressions are derived for path gain and for angular spectrum at the base in both urban and heavily foliated environments. The resulting predictions are in close agreement with widely accepted models and empirical results. The angular spectrum at the base in urban environments is found to be Lorentzian of width close to that reported for measurements in Aarhus. In foliated environments, vegetation is represented as statistically homogeneous diffuse scattering medium, resulting in a Gaussian-shaped angular spectrum at the base. View full abstract»

An empirical propagation prediction model is described for mobile communications from high altitude platforms (HAPs) in different types of built-up areas. The model introduced here is defined as a function of the angle of elevation. The target frequencies are selected from the 2 to 6 GHz frequency band prospective for 3G and 4G mobile systems, namely at 2.0,3.5, and 5.5 GHz. This new HAP model recognizes two cases - line of sight (LOS) and non-line of sight (NLOS) between a HAP and a user at street level. The simulation of the urban environment is based on a statistical approach. Additional shadowing path loss is calculated using the uniform theory of diffraction for NLOS conditions. Normal distribution of the additional shadowing path loss was distinguishable from the simulation results. The shadowing path loss is defined as a function of the elevation angle. The results of the empirical model developed for idealized conditions are verified by measurements taken from a remote-controlled airship in different types of urban environment. Close correlation was achieved between the theoretical model and the experimental data. The HAP elevation dependent shadowing model is easy to implement and can be used for realistic planning and simulations of mobile networks provided via HAPs in built-up areas. View full abstract»

Between February 1,1997 and January 31,1998, a 2.1 km vertically polarized 38 GHz communications link was studied in the subtropical city of Brisbane, Australia. According to the current ITU fading prediction method the link would be expected to experience a rainfall intensity of approximately 50 mm/h for 52.6 min during the year, when averaged over a 60 s integration time. This should have caused approximately 20 dB of fading for 52.6 min [1]-[3]. However the link actually experienced 20 dB of fading for 150.5 min, with fading of 37 dB exceeded for 53.5 min during the year. A rainfall intensity of 84 mm/h was exceeded for 64 min at the receive end, and a rainfall intensity of 72 mm/h was exceeded for 52 min at the transmit end of the hop. View full abstract»

A model is presented that extends the applicability of the propagation data collected during a long measurements campaign in a particular site and for a particular radiolink, to a generic link in a generic site. In fact, the objective of the model is to create a set of attenuation time series to be used in the design of advanced TLC systems. The model assumes that the marked differences of the rain statistics observed in various locations are, in first approximation, mainly due to the different frequency of occurrence of the various rainy events rather than to their shape. Starting from this assumption the model selects, from a large database of rain attenuation events collected in a particular site, a subset that, after an appropriate frequency and elevation scaling, can be used to represent time series of attenuation recorded in other experimental conditions (as for both site and link). The model has been applied to the attenuation data collected at Spino d'Adda (Milan, Italy) during the ITALSAT propagation campaign in order to simulate attenuation time series in several different links and sites. The model demonstrated good performance both in link conditions and climates, similar and different from the Spino d'Adda ones. View full abstract»

We review the model of Miller-Brown-Vegh (MBV) that is commonly used to describe scattering from the ocean surface. The model has been built on two essential elements: the Kirchhoff approximation and an assumption regarding the probability distribution of the ocean surface elevations. A comparison with theoretical results preceeding the MBV model as well as with experimental data suggest that the MBV model is inconsistent with the predicted and observed probability distribution of sea waves. Here, a scattering model based on more rigorous statistics of the ocean surface is considered instead. The analysis we present offers an explanation as well as a remedy for the reported tendency of models describing propagation in evaporation ducts to overestimate the signal's intensity at the receiver. View full abstract»

The diffusion behavior of electromagnetic (EM) waves in two dimensional (2-D) multipath media is studied through integral equation based full wave Monte Carlo simulations. The influences of some physical factors are explored, among which the area density of the embedded obstacles manifests itself to be the most important one in determining wave diffusion. A lossy system starts to behave diffusively when the area density approximately exceeds 5%, and the diffusion equations are generally applicable for predicting power decay. At low densities, the power-distance relation of the waves appears to follow power laws. The sizes and shapes of the obstacles have a secondary effect on the diffusion of waves. Whenever a system contains small objects or objects with reflecting sides, the waves therein are more diffusive and the diffusion equation approximates the reality more accurately. Absorption loss decreases wave diffusion in general, but our results show that the diffusion equation for a system with very lossy but small obstacles can work very well for predicting power decay. View full abstract»

A new set of higher order hierarchical basis functions based on curvilinear triangular patch is proposed for expansion of the current in electrical field integral equations solved by method of moments. The hierarchical two-level spectral preconditioning technique is developed for the generalized minimal residual iterative method, in which the multilevel fast multipole method is used to accelerate matrix-vector product. The sparse approximate inverse (SAI) preconditioner based on the higher order hierarchical basis functions is used to damp the high frequencies of the error and the low frequencies is eliminated by a spectral preconditioner in a two-level manner defined on the lower order basis functions. The spectral preconditioner is combined with SAI preconditioner to obtain a hierarchical two-level spectral preconditioner. Numerical experiments indicate that the new preconditioner can significantly reduce both the iteration number and computational time. View full abstract»